In recent years, due to the development of new media, even in general television sets, there is needed a system for enabling a single television receiver to receive an off-air TV signal (a TV signal which is broadcasted from a broadcasting station by radio waves), a CATV signal (a television signal which is transmitted through a cable), a satellite television broadcasting signal (a television signal which is broadcasted by radio waves through a broadcasting satellite), and the like. In such a case, generally both of the off-air television signal and the CATV signal are received by a single tuner and the satellite television broadcasting signal is received by another tuner and the system using those two tuners is provided in a television set.
FIG. 4 shows a construction of a BS tuner for receiving the satellite broadcasting signal. Reference numeral 21 denotes an input filter; 22 indicates a first IF amplifier; 23 a mixer; 24 a local oscillator; 25 a second IF amplifier; and 26 a prescaler.
In FIG. 4, the satellite television broadcasting signal is block converted into a first intermediate frequency (1.0 to 1.3 GHz) by a converter unit (not shown) provided for an SHF receiving antenna (parabolic antenna, plane antenna) and is input to an input terminal C. The input filter 21 is a band pass filter for allowing only the signal of the first intermediate frequency (hereinafter, abbreviated to BSIF) to pass. The BSIF signal is amplified by the first IF amplifier 22 including an automatic gain controller and is input to the mixer 23. In the mixer 23, the BSIF signal is mixed to an oscillation output from the local oscillator 24 and their differential frequency component is taken out as a second IF signal (134 MHz or 403 MHz). The second IF signal is amplified by the second IF amplifier 25 and is output from an output terminal D and is supplied to a frequency demodulator. On the other hand, a station selecting frequency control voltage from a station selecting control circuit is input to a terminal F' and is given to the local oscillator 24. The oscillation output from the local oscillator 24 is also supplied to the prescaler 26, by which it is frequency divided and supplied to a PLL frequency synthesizer circuit of the station selecting control circuit from a terminal E'.
FIG. 5 shows a construction of a conventional TV tuner for receiving a VHF/UHF TV broadcasting signal and a CATV signal. In FIG. 5, reference numerals 31 to 36 denote circuits to receive television signals of the VHF and CATV channels. Reference numerals 37 to 42 indicate circuits to receive a television signal of the UHF channel. In the VHF/CATV channel receiving circuit, reference numeral 31 denotes an input tuning filter; 32 indicates an RF amplifier including an automatic gain controller; 33 a double-tuned filter between stages; 34 a mixer; 35 a local oscillator; and 36 an IF amplifier including an automatic gain controller. In the UHF channel receiving circuit, reference numeral 37 denotes an input tuning filter; 38 an RF amplifier including an automatic gain controller; 39 a double-tuned filter between stages; 40 a mixer; 41 a local oscillator; and 42 a switching circuit.
In the television tuner of FIG. 5, the television signals of the VHF band and CATV channel which were input from the antenna to a terminal A are channel selected by the input tuning filter 31 and are amplified by the RF amplifier 32 including the automatic gain controller and are further band limiting by the double-tuned filter 33 between stages and are input to the mixer 34. On the other hand, a local oscillation signal from the local oscillator 35 is also input to the mixer 34. An IF signal having the frequency of the difference between both of those input signals is taken out and is amplified by the IF amplifier 36 including the automatic gain controller and is output to a terminal B. The output IF signal from the terminal B is supplied to a next VIF circuit and is AM detected.
Then, the television signal of the UHF band is input from the antenna to a terminal G and is channel selected by the input tuning filter 37 and is amplified by the RF amplifier 38 including the automatic gain controller. The amplified signal is further band limited by the double-tuned filter 39 between stages and is input to the mixer 34. On the other hand, a local oscillation signal from the local oscillator 41 is also input to the mixer 40. Those input signals are converted into an IF signal having the frequency of the difference between them and the IF signal is input to the mixer 34 through the switching circuit 42. In this case, the mixer 34 operates as an IF amplifier for the UHF television IF signal. The IF signal is further amplified by the IF amplifier 36 and is output to the terminal B. On the other hand, a station selecting frequency controlled voltage from the station selecting control circuit is supplied to a terminal F". Upon both of the VHF/CATV reception and the UHF reception, the same control voltage is supplied to the local oscillators 35 and 41, double-tuned filters 33 and 39 between stages, and input tuning filters 31 and 37. The television tuner system is of the single super type and almost of the general TV tuners are constructed by such a type.
As mentioned above, in the television set constructed so as to receive the TV signals of the general VHF, UHF, and CATV and the satellite television broad-casting signal, as its tuner section for reception, two kinds of tuners must be provided by individually respectively constructing the tuners shown in FIGS. 4 and 5. Thus, the circuit construction becomes complicated, and a large number of parts are needed, and the costs cannot be reduced.
In consideration of the above problems, it is an object of the present invention to provide a tuner for receiving a television signal in which the circuit construction can be simplified and the costs can be remarkably decreased.